Firefox's enhanced security measures can prevent you from using various plug-ins within Canvas, such as Zoom Client. To resolve this issue, you will need to turn off Enhanced Tracking Protections in Firefox.

To do this, log into Canvas in Firefox. Click on the shield icon. 

A little window with protection settings will appear. Toggle the "Enhanced Tracking Protection" button to OFF.

The Canvas page will refresh and then you will be able to access Zoom Client and other Canvas plug-ins without issue. 

The below picture shows how protections for Canvas should look.